To conduct our noise level tests, we replaced the graphics card in our system with a AMD Radeon HD 5550 Silence which was passively cooled. We also momentarily disconnected the fans on the CPU cooler so we were only taking into account the noise generated by the fans in the case.
This graph shows that the Obsidian 250D remains fairly quiet, even when under load. This is particularly impressive for a case with so many air vents!